💊 Carbimazole

Carbimazole is an antithyroid drug, a pro-drug that is metabolised to methimazole (thiamazole). It is used to treat hyperthyroidism.

Carbimazole's active metabolite, methimazole, inhibits the enzyme thyroid peroxidase. This enzyme is crucial for the organification of iodide and the coupling of iodotyrosines, thereby blocking the synthesis of thyroid hormones (T3 and T4) within the thyroid gland.

Primarily indicated for the treatment of hyperthyroidism, including Graves' disease, toxic multinodular goitre, and toxic adenoma. It can be used as a definitive treatment, to prepare patients for thyroidectomy or radioiodine therapy, or as a long-term 'block and replace' regimen.

Known hypersensitivity to carbimazole or other thionamides. Severe hepatic impairment is a contraindication. Pre-existing bone marrow depression or a history of carbimazole-induced agranulocytosis are absolute contraindications. Pregnancy, especially in the first trimester, requires careful consideration due to teratogenicity.

Common adverse effects include rash, pruritus, arthralgia, and nausea. The most serious and life-threatening adverse effect is agranulocytosis, which presents as fever, sore throat, or mouth ulcers. Other serious effects include hepatotoxicity and pancreatitis.

Full blood count (FBC) should be performed before treatment and patients must be advised to report any signs of infection (e.g., fever, sore throat) immediately for an urgent FBC. Liver function tests (LFTs) should also be monitored. Thyroid function tests (TSH, free T4, free T3) are monitored to assess treatment efficacy and guide dose adjustments.

Crucially, counsel patients extensively on the symptoms of agranulocytosis (fever, sore throat, mouth ulcers) and the need for urgent medical review and FBC if these occur. Always check baseline FBC and LFTs. Discuss teratogenicity in women of childbearing potential and the need for effective contraception.

High-yield for the risk of agranulocytosis and the associated patient safety advice. Remember its mechanism of action (inhibits thyroid hormone synthesis). Be aware of the preference for propylthiouracil in the first trimester of pregnancy due to carbimazole's teratogenicity, but carbimazole is generally preferred post-first trimester. Know common side effects like rash.

SBAs frequently test the most serious adverse effect, agranulocytosis, and the associated patient counselling (sore throat/fever). Questions may also focus on the mechanism of action or the use of carbimazole in pregnancy (avoid in first trimester if possible, propylthiouracil preferred). Expect scenarios involving a hyperthyroid patient developing a fever.